1. Field of the Invention
This invention relates to the transmission of periodic data in a real time data imaging network system, and more particularly, in such a system in which different data messages are repetitively transmitted at different periodic rates.
2. Background Information
In a real time or deterministic data imaging network system, data messages containing current images of specific data are periodically transmitted by stations on the network to the other stations. Data latency, that is the maximum time period in which the data must be updated at receiving stations, determines the periodic rate at which messages must be transmitted. Periodic rate in turn determines the amount of message traffic that must be handled by the system.
A network conforming to the FDDI (fiber distribution data interchange) standard is an example of a network which can be utilized as a real time data network. The FDDI protocol defines a ring topography and utilizes a token passing scheme in which a token is passed from station to station to allocate transmission time on the network. On an FDDI network, traffic is controlled by allocating messages to one of two classes: synchronous and asynchronous. The synchronous class of messages is used for periodic data, because it guarantees a maximum data latency equal to twice the target token rotation time (TTRT) where the TTRT is selected to achieve the required performance in the time domain. For example, if a maximum data latency of 100 milliseconds is required, the TTRT is selected to be 50 milliseconds. The TTRT then determines the maximum amount of traffic which can be broadcast on the network, because in the worst case, all on the network must be capable of broadcasting their synchronous mode messages and passing the token within the TTRT.
In some real time systems, such as for example, a real time control system, there are often data which must be updated at a high data rate for appropriate control. Normally, most of the data in the system has a much longer latency requirement, and can in fact have varying latency requirements. In such a non-homogeneous system, the capacity of the network is constrained by the periodic data having the smallest broadcast period. This is because the peak loading occurs when all nodes on the network simultaneously require the token to transmit all of their periodic data during a single token rotation.
There is a known real time token passing network system in which periodic data is transmitted at one of two rates, a fast rate (having a short period) and a slow rate (having a longer period). The short period data is transmitted during every token rotation. Each station individually, and without regard to the messages transmitted by the other stations, distributes its periodic data messages having the longer period over the number of short data periods contained within the longer data period. Thus, each station tries to average out its own messages of the longer period. The effectiveness of this scheme is related to the number and length of the messages being transmitted. For example, if, within each station, the number of messages with the longer data period to be transmitted is equal to the ratio of the longer data period to the shorter data period, an equal number of longer data period messages would be transmitted in each of the shorter data periods, and therefore, the message traffic would be fully averaged out. For a network having very short messages, such a scheme is usually effective for providing smoothing across the entire network. However, as message sizes increase, it becomes more unlikely that such averaging within a station will provide effective smoothing. In the worst case, each station may have only a single long data period message and, there can be no averaging. Since all of the stations are distributing their long period messages independently, in the worst case, each station could transmit its one long data period message in the same short period.
There is another known real-time token passing network protocol, known as FDDI, which permits periodic, or synchronous, data to be transmitted at one of many periodic rates. The FDDI protocol does not have any provisions for smoothing synchronous data of differing periods, either within a station or across the entire network. Thus, the worst case (peak loading) occurs when all synchronous data from all stations on the network transmit their synchronous mode messages in a single token rotation. The likelihood of such an occurrence is compounded by the fact that each station generally transmits only a few messages which are relatively large in size. This is a case because the FDDI network is a very high throughput data network (100 Megabits per second). In such a network, software processing times generally dictate the performance; although the network can sustain a very high transmission rate, it cannot sustain a very high message rate, since each message must be processed by software. Therefore, it is very likely that the messages from each station on the network are constrained to be a certain minimum size, and that individual data entities are grouped into a single message. As a result, an FDDI network having multiple synchronous data periods has very uneven loading, and also has reduced capacity, since the capacity is determined by the worst case synchronous data traffic in a single token rotation.
There is a need therefore for a system which more effectively distributes long period data in systems with multiple data periods.